1. Field of the Invention
The present invention relates to telecommunications cables. More particularly, the invention relates to cables containing optical fibers disposed in strippable tubular coverings or buffer tubes made of thermoplastic elastomer.
2. Description of the Related Art
Optical fiber cables generally include an outer thermoplastic sheath, which may include metal or dielectric protective reinforcing elements, that surrounds reinforcing or protective layers, that in turn surround a plurality of optical glass fiber constructions. The optical glass fiber constructions are generally of either a “loose-tube” construction or a “tight buffer” construction.
A cross-sectional view of an optical fiber cable with a loose tube construction is shown FIG. 1. Cable 10 has an outer jacket 12 that may incorporate one or more strength members 17. The jacket 12 is generally comprised of a protective polymer such as polyethylene or polyvinyl chloride. The strength members 17 are generally steel or high strength synthetic cables. The jacket 12 surrounds one or more layers 14 and 16 of reinforcing or protective materials. The layers 14 and/or 16 surround a plurality of optical fiber bundles 18. Reinforcing materials in the layers 14 and/or 16 may be formed as a flexible metal sheath and/or a high strength synthetic fiber sheath such as a covering comprised of Kevlar® aramid yarn. One protective material used in the layers 14 and/or 16 can be a layer of a super absorbent polymer. As best shown in FIG. 2, each optical fiber bundle 18 is comprised of anywhere from two to twenty-five optical fibers 20 that are encased within a buffer tube 19. Each buffer tube 19 provides both mechanical and chemical protection for the optical fibers, and the buffer tubes 19 may be color coded for identification purposes.
A perspective cut-away view of a cable having optical fibers in a tight buffer construction is shown in FIG. 3. An outer jacket 22 is provided that may incorporate strength members. The outer jacket 22 surrounds a plurality of individual sheathed fibers 24 that in turn surround a central strength member 25. The jacket 22 is generally comprised of the same type of polyethylene or polyvinyl chloride polymers found in the outer cable jacket 12 of the cable of FIG. 1. The central strength member 25 is typically a cable of steel or another high strength reinforcing material. As best shown in FIG. 4, each of the individual sheathed fibers 24 are comprised of an optical fiber 28 surrounded by an extruded polymeric buffer tube 27, that is surrounded by a reinforcing sheath 26, that is in turn surrounded by an outer individual fiber sheath 21. The optical glass fibers are frequently treated with acrylics or silicon to reduce the sensitivity of the glass fibers to surface damage. The tight buffer tube 27 is generally comprised of a thermoplastic elastomer, polyvinyl chloride, polyethylene or polypropylene. The reinforcing sheath 26 is generally comprised of a high strength synthetic fiber covering such as a covering comprised of Kevlar® aramid yarn. The outer individual fiber sheath 21 both protects and color codes the optical fibers, and it is often made of polyvinyl chloride.
The buffer tubes of both the loose tube and tight buffer constructions are generally made of a strong, yet flexible material that at the same time is strippable so as to enable the optical glass fibers to be accessed with the fingers or simple tools. At the same time, the buffer tubes must not readily melt during the cable manufacturing process or the optical fiber bundles will stick together making it difficult to select a desired fiber bundle. This is a concern because the outer sheath or other protective layers may be formed by extruding polyethylene or polypropylene around the fiber bundles at molten polymer temperatures that can locally be as high as 150° or 160° C. In addition, it is desirable that the buffer tubes be hard and stiff enough so that the tubes can be readily handled during the cable manufacturing process, as for example when a fibrous reinforcing sheath is wrapped around the buffer tube of a cable having a tight buffer construction. Finally, it is also preferred that the buffer tube in a tight buffer construction not be subject to shrinkage during the cable manufacturing process as such shrinkage results in attenuation of the optical signal being carried by the optical fiber.
U.S. Patent Application Publication No. US 2002/0001440 and U.S. Pat. No. 6,483,971 disclose optical fiber cables made with optical fiber bundles or modules having an extruded skin of a thermoplastic material having flexible diol segments. One of the thermoplastic materials disclosed for the extruded tubes of the modules is Hytrel® thermoplastic elastomer sold by E.I. du Pont de Nemours and Company, Wilmington, Del., USA (“DuPont”). U.S. Patent Application Publication No. US 2002/0001440 specifically references Hytrel® grade G3548L (with an initial resistance to tearing of 60 kN/m, a melting point of 156° C., and a Shore D hardness of 35) and an even softer Hytrel® grade HTR 8 351 NC-010 (with an initial resistance to tearing of just 20 kN/m, a tube melting point of just 140° C., and a Shore D hardness of 23). However, with melting points below 160° C., melting of the buffer tubes is likely to occur during the cable manufacturing process. Further the sub 40 Shore D hardness of these elastomers makes them too soft for efficient handling during the manufacturing process for certain type of cables.
There is a need for a buffer tube for optical fibers that is very easily strippable from the encased optical fibers, but that will not be subject to melting or sticking to other buffer tubes during the cable manufacturing process. There is a further need for a buffer tube comprised of a thermoplastic copolyether ester elastomer that is easily strippable, but that is significantly harder and stiffer than the buffer tube compositions disclosed in the prior art. Unfortunately, when thermoplastics are modified to achieve a higher melting temperature or to obtain increased hardness or stiffness, the material tends to become more shrinkage prone and more difficult to tear.